1. Field of the Invention
This invention relates to projection exposure apparatus and method and a projection optical system optimum for the projection exposure apparatus, which are used for producing micro devices, such as semiconductor integrated circuits, image pickup elements such as a CCD, liquid crystal displays, and thin film magnetic heads, with lithography techniques.
2. Background of the Related Art
In recent years, as circuit patterns of micro devices, such as semiconductor integrated circuits, have become more minute, a wavelength of an illumination light for exposure (exposure light) used in an exposure device, such as a stepper, has continuously become shorter. In other words, for the exposure light, a KrF excimer laser beam (wavelength: 248 nm) has become mainstream, instead of an i-line (wavelength: 365 nm) of a mercury lamp, which had been used conventionally. In addition, the use of an ArF excimer laser beam that has an even shorter wavelength has been achieved. Moreover, for the purpose of making the exposure light shorter, the use of, for example, a halogen molecular laser such as an F2 laser (wavelength: 157 nm) has been considered.
As an ultraviolet or vacuum ultraviolet light source, there are the above-described excimer laser, halogen molecular laser, and the like. However, materials that transmit radiated ultraviolet or vacuum ultraviolet beams are limited. Thus, only limited materials can be used for lens elements that structure the projection optical system, and the transmissivity of such limited material is not high. Currently, the performance of an anti-reflective coating provided on the surface of lens elements is not very high compared with that for longer wavelengths. Furthermore, recently, with the changes in illumination conditions in illumination optical systems and changes in environment of the projection optical system, it has been required to control the changes in image forming performance in the projection optical system.